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Overfilled tape cartidge

Posted by orik 
Overfilled tape cartidge
April 28, 2009 09:51AM
Hello all,

I am using an LTO-2 ADIC autoloader.
As you know, the tape has a 200 GB native capacity and 400 GB compressed.
How is it possible that my backup job is currently backing above 400 GB, it even backed up almost 500 GB without an error (at least not being logged).
What I want to know is how it is possible to reach over the limit of the tape and what are the consequences?
Why am I not being informed by Brightstor about this overload?
Does this mean that this backup is unreliable?

Overfilled tape cartidge
April 28, 2009 04:24PM
two tape capacities, native and compressed.

native - this value is engineered so that this amount of data can be written to the tape.

compressed - this value is set by the marketting heads, just a figure they thought would sound good.

The first value never changes, it is always the same regardless of if compression is or is not used. A 200/400 drive will always write 200gb to tape.

The second value changes all the time depending on the type of data read. Compressed data (jpg, mp3, zip, etc) will not compress. On the other hand text files will compress greatly. So you can fit 500gb or more on a 200gb tape.

Note: Understand this the native capacity is what is actually written on tape. The compressed capacity is what is read, compressed, and then sent to the head to be written.

Example: 500gb of data on disk, is read, compressed, then written to tape, and the drive then reports tape full. So what happened is 500gb of tape just happened to compress down to 200gb. The tape still holds the same 200gb, it is just how much the data will compress before being written.
Overfilled tape cartidge
April 30, 2009 06:09AM
Couldn't be clearer.

Thanks you very much.
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